Interdigital capacitor dielectric constant probe

An interdigital capacitor configuration is proposed as a probe for determining the dielectric constant of planar structures. The sensing area of the probe is only 10 × 2.5 mm
²
. The feeding structure of the interdigital capacitor is composed of a 50 Ohm microstrip line connected to a tapered microstrip line section. The feeding structure combined with the intedigital capacitor introduces a series resonant circuit. The effect of radiated power is minimized at the corresponding resonant frequency by appropriate design of the tapered section. Thus, at the resonance, almost all transmitted waves pass through the interdigital capacitor. By contacting the sample under test, the capacitance is increased as functions of the dielectric constant and the thickness of the sample, thus the resonant frequency is decreased. Parametric studies for changes of resonant frequency versus dielectric constant and sample thickness are presented. Through these results, calibration curves for the proposed probe are obtained.